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Type B rcd's

Discussion in 'Electrics UK' started by blup, 12 Jul 2020.

  1. blup

    blup

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    I am buying the Megger MFT1711 which appears to meet the needs of single phase domestic installations I will be working on, one of which is fitted with type B rcbo's.

    The Megger website indicates this model will not test type b rcd's.

    Is this correct i.e. will it test type b's for disconnection times on equipment using ac devices only as opposed to say electric vehicle charging?

    Or do I need to upgrade to the 1721 which is more expensive and less cheaply available second hand on ebay?

    Cheers

    Blup
     
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  3. flameport

    flameport

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    The 1711 doesn't test type B RCDs, the others do.

    Exceptionally unlikely that any domestic installation has Type B RCDs or RCBOs.
     
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  4. scousespark

    scousespark

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    Sorry, you need the 1721 to test a type b. You just have to take the hit. I’m assuming you are looking at EV. If so, this MFT gives you an advantage over competitors who can’t test these RCDs and any advantage is good to have.
     
  5. AndyPRK

    AndyPRK

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    There are type B mcbs. Very common.

    And in domestic. Type A and AC RCDs I believe.
     
  6. scousespark

    scousespark

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    Type B RCDs are specified for some EV chargers, so will be mor common in domestic installations in the near future.
     
  7. scousespark

    scousespark

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    Currently Megger have the 1711 as their entry level tester and this fine for domestic installers. As EV chargers become more common place, the 1721 will become the entry level tester. Megger may add type b testing to the 1711 (just a guess)
     
  8. Risteard

    Risteard

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    I have never come across an RCBO with Type B RCD. Only RCCBs (and used them). Type B won't} replace Type A as many EVSEs incorporate 6mA DC detection.

    I challenge anyone to find an RCBO which has Type B RCD protection.
     
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  9. ericmark

    ericmark

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    Beat me to it, yes I thought my RCBO's were type B that's what it said on the box, but it turned out they were curve B type AC. It seems common to say type B when they mean curve B.
     
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  11. blup

    blup

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    Thanks for helpful replies.

    Have just checked, they are Wylex NHXS rcbo's, the technical data says they are type A rcd's with type b and c tripping characteristics.

    Do the professionals have any preferences/recommendations for quality boards or ones to avoid? Should I get surge protection and/or arc fault protection?

    Blup
     
    Last edited: 18 Jul 2020
  12. scousespark

    scousespark

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    I had assumed you were an electrician. Surely, the electrician you refer to will have the required kindle(and know how to use it and interpret the results)
     
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  13. ericmark

    ericmark

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    I am uncertain even as an electrician as to type of RCD, there is a logo on the RCD/RBCO like this Type A.png which in this case shows it is type A with just wavy line without bumpy line under it is type AC seems many this is the only marking to show which type. It is reported there is a potential problem with type AC that if diode fail on power supplies it can raise the tripping current to above the rated 30 mA so with inverter drives and switch mode power supplies one should use type AC or better, but there are also reports to say in practice on testing there were no faults found, however some countries have banned the use of type AC.

    As to "Should I get surge protection and/or arc fault protection." again I really don't know, maybe with aerial feeds the surge protection is good, I fitted it to my house as it came with the board, as to arc fault protection with a wood frame house I would say again maybe, but we have had electrical supplies for years without them, and no real problems. I note they are on the best practice guide as items to check for with an electrical installation condition report, but with my brick built house I would not worry too much. It is a risk assessment with arc fault protection and also how many RCD's, unlike the MCB and surge protection which never tends to trip in error an AFDD or RCD can trip in error, this can plunge you into darkness, or cause the loss of a freezer full of food, so having a board of RCBO's instead of two RCD's means they are less likely to trip in error, and if they do trip only one circuit is affected. It also means the board is shorter, as RCD's take up two slots each. But my all RCBO's board cost over £200 for 14 way, and that was cheap, I miss read the spec and got type AC RCBO's instead of type A likely with type A a 14 way board will cost over £250 before the fitting, however if you look at the cost of a freezer full of food, that is not that expensive, last house we had to dump two freezers full of food as the RCD tripped when we were away, modern freezers well at least mine on power returning shows how warm it got, so even if power lost and restored without your knowledge it shows what the highest temperature was. However I have not found anything to say food needs dumping if it reaches x°C at any time, and unlike bringing food home in the car it takes some time for temperature to both rise and fall inside a freezer, so if it shows -1°C so still frozen question is how long it has been at -1°C?

    The other point is with a genuine trip, how do you keep the freezer powered while you find the cause? To plug in to the cooker supply, or even run an extension lead to other side of the house will keep things running with reasonable safety until fault is found, however running an extension lead up/down the stairs is clearly a hazard specially with no lights, so if sockets split side/side or front/back then two RCD's can be used and there is a safe way to in an emergency get power to a freezer, but up/down often there is no safe method. The idea is lights in any one room should not be powered by same RCD as sockets in that room, so if there is an accident which trips sockets your not also plunged into darkness, if as with this house sockets split front/back and lights split up/down then this can't be arranged with just two RCD's.

    The regulations do not say use two RCD's, as clearly with emergency lights there is no danger, and if your prepared to dump a freezer full of food then no risk, and we are told one should test the leakage, seem to remember 9 mA is the limit for all circuits on a 30 mA RCD which can trip between 15 mA and 30 mA, so if the electrician actually measures first then a RCD should not trip without reason, but read this forum and clearly they do.

    So you assess the risk, if you feel you want to pay extra and have all RCBO protection, or some RCBO protection with a board with three neutral bars that is up to you, the electrician may say the back ground leakage is too high, you need more than two RCD's but in the main they offer the cheapest option, which may not be the best option, and if the board uses RCBO's at £26 each instead of £16 that can result in a rather expensive board, it is all well and good saying I like MK consumer units, but the metal consumer units have only been made in any quantity for some 3 years or so, so really no one has had time to find fault with certain makes of consumer unit, OK some may be harder to wire, but as user that does not really matter to you, so may as well open the book blind folded and stick in a pin.
     
  14. JohnW2

    JohnW2

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    Maybe historically, but my understanding is that MK stopped producing consumer units last year!

    Kind Regards, John
     
  15. JohnW2

    JohnW2

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    The terminology being used is potentially extremely confusing for RCBOs (particularly "Type B" RCBOs), since the word "Type" is being used with two totally different meanings.

    There is no problem with MCBs. "Type B", "Type C" and "Type D" refer unambiguously to the over-current tripping characteristics, in particular the current required to result in a magnetic trip - i.e. this "Type" refers to over-current characteristics.

    There is no problem with RCDs etc. "Type A", "Type AC", "Type B" and "Type F" refer unambiguously to the sensitivity of the device to different waveforms of residual current - pure AC, pulsed, with a DC component etc. - i.e. this "Type" refers to residual current characteristics

    It is with RCBOs (and specifically "Type B" RCBOs) that the problem arises, since one needs to define both of those characteristics for an RCBO. Hence, as eric has found to his cost, if one sees and RCBO with just "Type B" written on the packet, then one has to look closer at the documentation to discover exactly what one has got!

    There is, as far as I can see, no actual ambiguity with "Types" A, AC, C, D and F - since (to the best of my knowledge) those terms can only apply to one or other of the types of characteristic (C & D to over-current characteristics; A, AC and F to residual current characteristics) - but I would personally say that it would nevertheless be far less potentially confusing if the term "Type" were not used for both!

    Kind Regards, John
     
  16. JohnD

    JohnD

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    I have been turning this question over in my mind, because my house is fitted with Memshield 2 RCBOs which incorporate the MR30 pod. Around 20 years old now, I think. They were quite expensive so I was sad to think I might need modernisation. The MCB portions are all type B

    For anyone not familiar, the Memshield 2 RCBO is assembled, in the factory or in the field, by snapping an RCD pod onto a MCB. They are made to fit together, and this enables a very wide range of options to be built.

    I seem to recall, that they have pulsating DC protection, but do not seem to be listed on a search of "Eaton" and I don't have the technical literature from the time.

    I found a spec of the MR30 on a distributors website that says "tripping class A" which is what we now prefer.

    Can anyone more familiar with the question advise if they are still considered suitable?
     
  17. JohnW2

    JohnW2

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    From my limited understanding, I think you will find that Type B is 'even better' than Type A (which we 'now prefer' over Type AC). As I understand it, Type AC is designed for sinusoidal AC only, Type A extends that to also include 'pulsating DC' and Type B extends that even further to include, for example, 'constant' DC components. There is also a Type F, about which I know even less. Quoting from some MK RCD documentation ...
    I would therefore think that you should not be concerned about your residual current modules being Type B, per se (but might be concerned about their age), and probably should not consider changing them to Type A (which would probably be a retrograde step).

    Kind Regards, John
     
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